Optical fiber fabrication

ABSTRACT

A preform for fabrication of a glass fiber optical transmission line is prepared by chemical reaction of vapor ingredients within a glass tube. Reaction, which may be between chlorides or hydrides of, for example, silicon and germanium with oxygen, occurs preferentially within a constantly traversing hot zone. Flow rates and temperature are sufficient to result in glass formation in the form of particulate matter on the inner surface of the tube. This particulate matter deposits on the tube and is fused with each passage of the hot zone. Continuous rotation of the tube during processing permits attainment of higher temperatures within the heated zone without distortion of the tube.

This is a continuation of application Ser. No. 828,617, filed Aug. 29,1977, now U.S. Pat. No. 4,217,027, which is a continuation ofapplication Ser. No. 444,705, filed Feb. 22, 1974, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is concerned with fibers for use as transmission lines incommunications systems operating in the visible or near visible spectra.Such fibers are generally clad for guiding purposes so that refractiveindex decreases in value from the core center to the periphery either asa step function or as a continuous gradient.

2. Description of the Prior Art

"Optical" communications systems, that is systems operating in thevisible or near visible spectra, are now at an advanced stage ofdevelopment. In accordance with the view held by many, commercial usemay be expected within a period of about five years.

A system most likely to find initial, and probably long term, useutilizes clad glass fibers as the transmission medium. These fibers,generally having an overall cross-sectional diameter of about 100 μm,are generally composed of at least two sections: core and cladding. Thecladding layer is necessarily of lowered refractive index relative tothe core with typical index variation from core to clad being in therange from about 0.01 to 0.05. Structures under study may be single modeor multimode. The former is characterized by a sufficiently small coresection to efficiently accommodate only the first order mode. Suchstructures may have a core about 1 or 2 μm. Multimode lines typicallyhave core sections from 50 μm to 85 or 90 μm in diameter.

Multimode structures appear to be of somewhat greater interest at thistime. The greater core section facilitates splicing and permits moreefficient energy coupling to source and repeater. Introduction of manymodes into or, alternatively, generation of many modes within the linedoes give rise to a dispersion limitation which takes the form of asmearing due to the differing velocities of different order modes. Modedispersion effects have been minimized by a continuous focusingstructure. This structure takes the form of a fiber, the index of whichis graded generally exponentially from a high value at the core center.The fundamental mode which traverses the length of material is generallyconfined to the highest index (lowest velocity) region, while higherorder modes as path length increases spend longer and longer periods inrelatively low index (high velocity) regions.

A number of procedures have been utilized for fabricating clad glassfibers. Most have yielded to procedures which in some way involve vaporsource material. So, typically, chlorides, hydrides, or other compoundsof silica, as well as desired dopants, tailoring the index, are reactedwith oxygen to produce deposits which directly or ultimately serve asglass source material from which the fiber is drawn. Dopant materialsinclude compounds of, for example, boron for lowering index andgermanium, titanium, aluminum, and phosphorus for increasing index.Where the ultimate product is to be a graded multimode line, indexgradation may be accomplished, for example, by altering the amount ortype of dopant during deposition.

One procedure utilizing vapor source material is chemical vapordeposition (CVD). In this procedure, compounds are passed over a heatedsurface--e.g., about a rod or within a tube. Temperatures and rates areadjusted so that reaction is solely heterogeneous, i.e., occurs at theheated surface so that the initial material is a continuous glass layer.

An alternative procedure results in the introduction of such precursormaterials into a flame produced by ignition of a gaseous mixture of, forexample, methane and oxygen. Reaction is, in this instance, homogeneousresulting in formation of glassy particles within the flame. Combustionproduct and glassy particles then form a moving gas stream which is madeincident again on a heated surface, such as a rod or tube. Adherentparticles sometimes called "soot" are in subsequent processing flushed,and are sintered and fused to result in a glassy layer.

The CVD process has advantages including high purity but suffers fromprolonged required deposition periods. Typically, a suitable preformadequate for fabrication of a kilometer of fiber may require periods ofa day or longer.

The soot process has the advantage of high speed; preforms adequate forfabrication of a kilometer of fiber may be prepared in a few hours orless. Disadvantages, however, include at least initial introduction ofcontaminants, such as solid carbonaceous residue. Since formation takesplace within the combustion environment, hydration is inevitable; andthis gives rise to the wellknown water absorption peaks with theirrelated subharmonics so consequential in various portions of theinfrared spectrum.

Both procedures are now an established part of the art. See, forexample, U.S. Pat. Nos. 3,711,262, 3,737,292, and 3,737,293.Modifications in the processes have, to some extent, increased the speedof the CVD process and reduced the effects of contamination by hydrationin the soot process. Fibers a kilometer or more in length with losses aslow as 2 or 3 dB/kilometer in selected regions of the infrared are nowregularly produced in pilot operations.

SUMMARY OF THE INVENTION

The invention provides for fabrication of clad glass fibers by aprocedure which combines some of the advantages of the prior art CVD andsoot processes. Generally, gas phase precursor reactants together withoxygen are introduced into a glass tube in the form of a constantlymoving stream. Tube and contents are heated to homogeneous reactiontemperature within a moving hot zone produced by a moving heating meansconstantly traversing the outside surface of the tube. Homogeneouslyproduced glass particles ("soot") collect on the tube walls, and arefused into a continuous layer within the moving hot zone.

With usual heating means there is simultaneous heterogeneous reaction sothat a glassy layer is produced within the moving hot zone by reactionat the heated wall surface. This deposit, which is present underordinary circumstances, is identical to the layer produced in the normalCVD processing.

In accordance with the preferred embodiment, the tube within whichformation is taking place is continuously rotated about its own axis.For example, at a speed of 100 rpm, uniformity about the periphery isenhanced. The surface produced by the molten CVD layer may help to holdthe "soot" particles during fusion.

Reactant materials include chlorides and hydrides, as well as othercompounds which will react with oxygen as described. As in other vaporreaction processes, other gaseous material may be introduced, forexample, to act as carrier or, in the instance of extremely combustiblematter such as hydrides, to act as a diluent.

Continuous fusion within the hot zone and the resultant thicknessuniformity of deposit facilitates formation of graded index structures.As in CVD, gradients may be produced by varying reactant compositionwith the ratio of high index-producing dopant increasing, in thisinstance, with successive hot zone traversals. Since reaction conditionsfor different constituents in the reactant mix are different, it ispossible also to produce a gradient by altering temperature and/or flowrate during processing.

Typical reaction temperatures maintained at least at the tube wall arewithin the range of from 1200 to 1600 degrees C. These temperatures,high relative to CVD, are responsible for rapidity of preformproduction. Particularly at the high temperature end of the range,distortion of the usually silica tube is avoided by rotation. Narrowzones, increased rotation speed, and vertical disposition of the tubemay all contribute to the avoidance of tube distortion.

Preforms adequate for preparation of one or a few kilometers of fibermay be prepared during deposition periods of one or a few hours. Thesepreforms are prepared by conventional processing from the depositedproduct to a final configuration which, as presently practiced, may beof rod shape with an internal diameter of from 4 to 8 mm and a length of18 inches. In usual processing, the tube which served as the depositionsubstrate becomes the clad. It may, in accordance with the system, becomposed of pure silica or of silica which has been doped to alter,generally to reduce its index. Variations may include removal of thetube, as well as deposition of additional material on the outer surface.The tube serving as the substrate during deposition may be retained toserve as a clad, may be removed, or may, during simultaneous deposition,on its outer surface be provided with encompassing layer/s.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a front elevational view of apparatus suitable for practice ofthe deposition process in accordance with the invention;

FIG. 2 is a front elevational view of apparatus alternative to that ofFIG. 1;

FIG. 3 is a front elevational view of a section of tubular materialdepicting observed conditions during processing; and

FIG. 4, on coordinates of insertion loss in units of dB/kilometer andwavelength in nanometers, is a plot showing the relationship of thosetwo parameters for a clad multimode fiber produced in accordance withthe invention.

DETAILED DESCRIPTION 1. The Drawing

FIG. 1 depicts a lathe 1 holding substrate tube 2 within which a hotzone 3 is produced by heating means 4. Tube 2 may be rotated, forexample, in the direction shown by arrow 5a by means not shown and hotzone 3 is caused to traverse tube 2 by movement of heating means 4 asschematically depicted by double headed arrow 5b, for example, by athreaded feed member 6. A gaseous material is introduced into tube 2 viainlet tube 7 which is, in turn, connected to source material reservoirs8. Such reservoirs may include an oxygen inlet 9 connected to means notshown. As depicted, gaseous material may also be introduced by inlets 10and 11 by means not shown and through inlet 12 from reservoir 13.Reservoirs 14 and 15 contain normally liquid reactant material which isintroduced into tube 2 by means of carrier gas introduced through inlets10 and 11 with the arrangement being such that the carrier gas isbubbled through such liquids 16 and 17. Exiting material is exhaustedthrough outlet 18. Not shown is the arrangement of mixing valves andshut off valves which may be utilized to meter flows and to make othernecessary adjustments in composition. The apparatus of FIG. 1 isgenerally horizontally disposed.

The apparatus of FIG. 2 is, in its operational characteristic, quitesimilar to that of FIG. 1. Vertical disposition of the substrate tube,however, lends stability to the portion of the tube within the hot zoneand may permit attainment of higher temperature or of longer hot zonesin the traversal direction without objectionable distortion. Apparatusdepicted includes tube 20 which may optionally be provided with rotationmeans not shown. This tube is secured to the apparatus by means ofchucks 21 and 22 and a traversing hot zone is produced within tube 20 bymeans of a ring burner 23 which is caused to constantly traverse tube 20in the direction depicted by double headed arrow 24 by moving means 25.Gaseous material, for example, from source such as 8 of FIG. 1 isintroduced via inlet tube 26 and exiting material leaves via exhaust 27.

FIG. 3 is a front elevational view of a section of a substrate tube 30as observed during deposition. Depicted is a heating means 31 producinga hot zone 32 which is traversing tube 30 in the direction shown byarrow 33 by means not shown. Gaseous material is introduced at the leftend of tube 30 and flows in the broken section of the Figure in thedirection shown by arrow 34. For the processing conditions, which withrespect to traversal direction and hot zone temperature are those ofExampe 1, two regions are clearly observable. Zone 35 downstream of hotzone 32 is filled with a moving powdery suspension of particulate oxidicmaterial, while region 36, devoid of such particulate matter, definesthe region within which fusion of deposited material is occurring.

FIG. 4 is a plot for measured loss in units of dB/kilometer as measuredon 713 meters of fiber prepared in accordance with an Example herein.Abscissa units are wavelength in nanometers. It is seen that loss is ata minimum of about 2 dB/kilometer for the wavelength range of about 1060to 1100 nm (the limiting value on the plot). The peak at about 950 nm,as well as those at 880 and 730 nm, are characteristic sub-harmonics ofthe fundamental water absorption.

2. Processing Requirements

a. Reaction Temperature

Superficially, the inventive technique resembles conventional chemicalvapor deposition. However, whereas CVD conditions are so arranged thatdeposition is solely the result of heterogeneous formation at a heatedsubstrate surface, procedures of this invention rely upon significanthomogeneous reaction. In general, 50 percent or more of reaction productis produced in a position removed from substrate surface and results inthe formation of solid oxidic particles of the desired glasscomposition. These particles are similar to those produced during the"soot" process.

Homogeneous reaction is the result of sufficient rate of reactantintroduction and sufficient reaction temperature. Such conditions may beachieved simply by increasing one or both parameters until homogeneousreaction is visually observed. To optimize the process from thestandpoint of reaction, high temperatures are utilized. For the usualsilica based systems which comprise the preferred embodiment,temperatures at least at the substrate wall are generally maintained ata minimum of 1200 degrees C. at the moving position corresponding withthe hot zone. Maximum temperatures are ultimately limited by significantwall distortion. For horizontally disposed apparatus, such as that shownin FIG. 1, in which a hot zone of the length of approximately 2 cm movesat the rate of about 45 cm/min within a tube rotated at the rate ofabout 100 rpm, a temperature of 1600 degrees C. may be produced withoutharmful tube distortion. Decreasing the length of the hot zone,increasing the rate of rotation, increasing reactant flow rate, verticaldisposition of the tube, are all factors which may permit use of highermaximum temperatures without variation in tube geometry. Indicatedtemperatures are those measured by means of an optical pyrometer focusedat the outer tube surface. It has been estimated that for typicalconditions the thermal gradient across the tube may be as high as 300degrees C.

b. Flow Rates

This parameter, like temperature, is dependent upon other processingconditions. Again, a minimum acceptable rate for these purposes may bedetermined by visual observation. Highest flow rates are for thosematerials which by virtue of combustibility, high vapor pressure, etc.,are diluted to a significant extent by inert material. Examples are thehydrides where dilution frequently is as high as 99.5 volume percentbased on the total reactant content may necessitate a linear flow rateof at least 1 meter per second. Chlorides, which do not present thisproblem, need not be diluted to avoid combustion. Inert material, suchas nitrogen or helium, is introduced solely for transfer purposes andneed be present only in amount typically of up to 10 percent by volume.Flow rates are, as indicated, temperature dependent, with the requiredhomogeneous reaction taking place at acceptable rate only by an increaseflow of about 50 percent for each hundred degree increase in reactiontemperature.

c. Reactants

Examples were carried out using chlorides and hydrides. Other gaseousmaterials of sufficient vapor pressure under processing conditions whichreact with oxygen or oxygen bearing material to produce the requiredoxidic glass may be substituted. In a typical system, the substrate tubeis silica--generally undoped. Where this tube is of ordinary purity,first reactant introduced may be such as to result in the formation of afirst layer of undoped silica or doped with an oxide such as B₂ O₃ whichserves to lower the refractive index, which acts as a part of the cladand presents a barrier to diffusing impurity from the tube. It may beconsidered that, under these circumstances, the substrate tubeultimately serves as a mechanical support rather than as an opticalcladding. Subsequent to formation of this first barrier layer or absentsuch procedure, where the tube is of sufficient purity, reactantmaterials of such nature as to result in the desired index-increasedcore are introduced. In a chloride system, these may take the form of amixture of SiCl₄ together with, for example GeCl₄, and oxygen. Chloridesof other index increasing materials, such as phosphorus, titanium, andaluminum may be substituted for GeCl₄ or admixed. BCl₃ may also beincluded perhaps to facilitate glass formation because of lowered fusiontemperature; or because of refractive index lowering, the initialmixture may be altered during successive hot zone traversals so as toincrease index (by increasing GeCl₄ or other index-increasing dopantprecursor or by decreasing BCl₃).

Since the usual vapor phase glass precursor compounds are not oxidic,oxygen or a suitable oxygen bearing compound is generally included toform the ultimate oxidic glass. A satisfactory procedure, followed inexemplary procedures, takes the form of an oxygen stream bubbled throughreservoirs of liquid phase glass forming compounds. In one procedure,for example, oxygen streams were bubbled through silicon tetrachloride,and through germanium tetrachloride. These streams were then combinedwith vapor phase boron trichloride and additional oxygen, the resultantmixture being introduced into the reaction chamber.

Relative amounts of glass forming ingredients are dependent upon avariety of factors, such as vapor pressure, temperature, flow rate,desired index, etc. The appended examples indicate suitable amounts forproducing the noted indices under the noted conditions. Variants areknown to those familiar with glass forming procedures.

A variety of diluent materials may be utilized for any of the notedreasons so, for example, argon, nitrogen, helium, etc., may serve tomaintain desired flow rates to prevent precombustion, etc. Oxygenbearing compounds which may replace oxygen in whole or in part includeN₂ O, NO, and CO₂.

In general, concentration of 3d-transition metal impurities in the gasstream is kept below 10⁻² percent, although further reduction in lossaccompanies reduction of those impurities down to the part per billionrange. Such levels are readily available from commercial sources or bypurification by means similar to those taught by H. C. Theuerer, Pat.No. 3,071,444. As compared with the usual soot process, the inventiveprocedure is carried out in a controlled environment without directexposure to combustion products. This inherently results in avoidance ofinclusion of particulate combustion products. Where desired, hydrationresulting from combustion in the soot process may be minimized. This isa particularly significant advantage for operation in several portionsof the infrared spectrum which suffers from sub-harmonics of thefundamental H₂ O absorption. Water vapor may, therefore, be aparticularly significant impurity and, for many purposes, should be keptto a level below a few ppm by volume.

3. General Procedure

The procedure described is that which was followed in Examples 1 through4. Deposition was carried out within a 12 I.D. by 14 O.D. mm silicatube. The tube was placed on a glass lathe within which it was rotatedat 100 rpm. Before introduction of reactants, it was flushed with acontinuous stream of oxygen while traversing with an oxyhydrogen burnersufficient to bring the wall temperature to 1400 degrees C. The purposewas to remove any volatile impurities on the inside wall of the tube.

Following a period of 5 minutes, a mixture of oxygen, SiCl₄, and BCl₃replaced the oxygen flow. The composition of approximately 10 percentSiCl₄, 3 percent BCl₃, remainder oxygen, maintaining temperature at 1400degrees C. within the moving hot zone as measured at the wall. In thisparticular example, the zone was moved at a speed of approximately 45cm/min in the forward direction (direction of gas flow) and was rapidlyreturned to its initial position (approximately 30 sec. elapsed time tothe beginning of the slow traversal).

Formation of flaky material within the tube, at a position spaced fromthe wall generally downstream of the hot zone, was visually observed. Itwas deduced and verified that homogeneous reaction was largely withinthe zone with particulates being carried downstream by the moving gas.Deposition was continued for approximately twenty minutes followingwhich flow of chloride reactants was discontinued. Oxygen flow wascontinued for several passes of the hot zone.

The procedure to this point results in deposition of a layer serving ascladding. Core material was next deposited by introduction of SiCl₄ andGeCl₄. These reactants, too, were introduced with an oxygen carrier, asbefore. With the temperature of the hot zone increased somewhat to about1450 degrees C., deposition was continued for about one hour.

In this particular example, tube collapse was initiated with reactantsstill flowing simply by reducing the rate of traverse of the hot zone.This resulted in a temperature increase which ultimately attained alevel of about 1900 degrees C. to produce nearly complete collapse.Reactant flow was then stopped with final collapse producing a finishedpreform consisting of a GeO₂ --SiO₂ core with a borosilicate claddingsupported, in turn, by a silica layer. It will be recognized by thoseskilled in the art of fiber drawing, that the tube, without first beingcollapsed, can also be drawn into acceptable fiber. The resultingpreform was then drawn to result in a fiber having an overall diameterof approximately 100 μm with a core area defined as the region withinthe borosilicate layer having a diameter of approximately 37 μm. Thelength of fiber drawn was approximately 0.7 km. The method described insome detail in N. S. Kapany, Fiber Optics Principles and Applications(Academic Press, New York) (1967) pages 110∝117, involved the localheating of an end of the preform which was affixed to the fiber, whichwas, in turn, drawn at a constant velocity of approximately 60meters/min by winding on a 30 cm diameter mandrel rotating at 60 rpm.

The above description is in exemplary terms and is usefully read inconjunction with the appended examples. The inventive process departsfrom conventional CVD as described--i.e., in that reactant introductionrate and temperature are such as to result in homogeneous reaction toproduce oxidic particles within the space enclosed, but separated fromthe walls of a tube. This, when combined with a moving hot zone, resultsin rapid preparation of a high quality preform as described. The movinghot zone is responsible for (1) homogeneous reaction; (2) to a largeextent, the adherence of oxidic particles to the wall; and (3) fusion ofthe deposited particles and CVD-produced layer into a unitary,homogeneous glassy layer. In general, it is desirable to maintain thehot zone as short as possible depending upon constancy of traversalspeed to result in uniform layer production. Motion of the hot zoneshould be such that every portion of the tube is heated to the zonetemperature for the same period of time. This is easily accomplished bypassing the heating means through a traversal distance which extendsbeyond the tube at both ends. Experimentally, hot zones of the order of2 cm length (defining the heated region extending 4 cm on either side ofthe peak) have resulted in uniform coating under all experimentalconditions. While, in principle, heating the entire tube may result inuniformity of deposition approaching that attained by use of a movingzone, very high flow rates are required to avoid inhomogeneity anddiffering thickness of deposit along the length of the tube.

4. Examples

The following examples, utilizing chloride or hydride reactants, are setforth. The selection was made with a view to demonstrating a widevariety of compositions and different types of optical waveguidepreforms for which the procedure can be used.

The tube of commercial grade fused quartz was first cleaned by immersionin hydrofluoric acid-nitric acid solution for three minutes and wasrinsed with deionized water for a period of one hour. Tubing was cutinto 18" lengths, and such sections were utilized in each of theexamples. The substrate tube was provided with appropriate input andexhaust sections, and was heated with a moving oxyhydrogen torchproducing a hot zone which traversed the tube in from one to eightminutes. In each instance, flushing was by oxygen at a flow rate ofbetween 100 and 500 cm³ /min corresponding with a linear rate of 4.5meter/mins, and this flushing was continued for several traversals ofthe zone.

EXAMPLE 1

The fused quartz tube used in this example was 12 mm I.D.×14 mm O.D.Initial deposition was of a cladding material, SiO₂ --B₂ O₃, byintroduction of 41 cm³ /min. SiCl₄, 12.5 cm³ /min BCl₃, both carried byoxygen such that the total oxygen flow was 250 cc/min. Sixteen passes ofthe hot zone were made at a temperature of 1430 degrees C. Core materialwas next deposited by flows of 32 cc/min SiCl₄, 48 cc/min GeCl₄, andoxygen 650 cc/min. This was continued for 68 minutes and temperatures ofthe hot zone were maintained at 1460 degrees C. Remaining steps,including partial collapse with flowing gas and final collapse under noflow conditions, were as specified under Section 3. The fiber thatresulted from this procedure had a core of approximately 40 μm with anoverall diameter of approximately 100 μm. Its length was 723 meters andoptical attenuation was 2 dB/km at 1060-1100 nm.

EXAMPLE 2

A fused quartz tube 6 mm I.D.×8 mm O.D. was cleaned as described andpositioned in a glass lathe. Flows of diluted (1 percent by volume inN₂) silane, germane, diborane, and oxygen were passed through the tubeas follows:

    ______________________________________                                               SiH.sub.4   1,000 cc/min.                                                     GeH.sub.4     150 cc/min.                                                     B.sub.2 H.sub.6                                                                             50 cc/min.                                               ______________________________________                                    

Deposition commenced by heating the tube locally using the oxyhydrogenflame which was traversed along the length of the tube. The completecycle took 3.7 minutes, and the highest temperature attained was 1400degrees C. After 175 minutes, the gas flows were stopped and the tubecollapsed in one additional pass, made at a much slower rate.Temperatures achieved here were in the vicinity of 1750-1900 degrees C.The preform was removed to a pulling apparatus and drawn to a fiberwhose diameter was 100 microns overall. This consisted of a core whosecomposition was SiO₂ --GeO₂ --B₂ O₃ of approximately 25 micronsdiameter. The cladding had the composition of SiO₂. The index differenceproduced by the core was 0.007.

EXAMPLE 3

A clean fused silica tube 6 mm I.D.×8 mm O.D. was positioned in a glasslathe as previously described. Flows of diluted (3.05 percent by volumein N₂) silane, diborane, and oxygen were passed through the tube asfollows:

    ______________________________________                                               SiH.sub.4    295 cc/min.                                                      B.sub.2 H.sub.6                                                                             49 cc/min.                                                      O.sub.2      900 cc/min.                                               ______________________________________                                    

Deposition commenced by heating the tube locally using an oxyhydrogentorch which traversed along the tube at a rate of 0.10 cm/sec as thetube rotated at 100-120 rpm. The torch was adjusted so as to produce atemperature locally of 1375-1450 degrees C. When the torch had moved tothe end of the tube, it was returned at 0.15 cm/sec with the SiH₄ and B₂H₆ flows stopped. This procedure continued for three hours. At this timethe B₂ H₆ flow was stopped and just SiH₄ and O₂ continued. At the sametime, the torch was adjusted to produce temperature of 1600-1650 degreesC., other conditions remaining the same as previously. Depositing thepure SiO₂ layer continued for 1.5 hours.

At this time, silane flow was stopped and just O₂ flow continued at 600cc/min. Temperatures were varied during the next two passes to 1650-1700degrees C. Now the oxygen was stopped, the traverse slowed to 0.05cm/sec, and the temperature raised to 1850-1890 degrees C. to bringabout complete collapse of the tube.

This procedure produced a preform having a core of pure SiO₂, a claddinglayer of B₂ O₃ --SiO₂, and an outer jacket of commercial grade SiO₂. Thefiber drawn from this preform had a core of 30 μm, cladding thickness of15 μm and an outer jacket of 20 μm, with an index difference of 0.007percent and losses of 3 dB/km at 1.06 μm wavelength.

EXAMPLE 4

For optical communications employing multimode optical fibers it isdesirable to more nearly equalize the group velocities of propagatingmodes. This result is expected if the index of the core is graduallyincreased from the cladding toward the interior of the core. Toaccomplish this a 8 mm I.D.×10 mm O.D. fused quartz tube was positionedand borosilicate layer intended to serve as a portion of the claddingand as a barrier layer was deposited as in Example 1. Next deposition ofthe GeO₂ --B₂ O₃ --SiO₂ core was commenced except that the germaniacontent was gradually increased from zero during the period ofdeposition. The conditions used during the deposition were as follows:

    ______________________________________                                        Barrier layer                                                                 SiCl.sub.4           32 cc/min                                                BCl.sub.3            12.5 cc/min                                              O.sub.2              250 cc/min                                               Temp                 1740 degrees C.                                          Time                 25 min.                                                  Graded Index portion of the core                                              SiCl.sub.4           33 cc/min                                                BCl.sub.3            12.5 7.5 cc/min                                                               17 equal increments at                                                        2 min intervals                                          GeCl.sub.4           0-35 cc/min                                                                   17 equal increments at                                                        2 min intervals                                          O.sub.2              460-830 cc/min                                                                17 equal increments at                                                        2 min intervals                                          Temp                 1470 degress C.                                          Constant Index portion of core                                                SiCl.sub.4           32 cc/min                                                BCl.sub.3            7.5 cc/min                                               GeCl.sub.4           35 cc/min                                                O.sub.2              830 cc/min                                               Temp                 1470 degrees C.                                          Time                 53 min.                                                  ______________________________________                                    

At the conclusion of the deposition, the tube was collapsed to yield asolid preform which was then pulled to yield an optical fiber. When themode dispersion of this fiber was measured, it behaved in a mannerexpected of a graded index. This behavior can be expressed by relation(Bell System Technical Journal 52, pp. 1566 (1973)) η=η₀[1-2Δ(r/a).sup.α ]^(1/2) where in this instance the value of α=5.

What is claimed is:
 1. A process for fabrication of an optical fibertransmission line comprising a core section and a cladding, wherein thecladding has an index of refraction of a value lower than the maximumindex of the core for energy of the wavelength to be transmittedcomprising:introducing a moving stream of a vapor mixture including atleast one compound glass-forming precursor together with an oxidizingmedium into a tube while heating the tube so as to react the saidmixture and produce a glassy deposit on the inner surface of the tube,characterized in that the heating of tube and contents are by a movinghot zone produced by a correspondingly moving heat source external tothe tube, and in that temperature within the hot zone, composition ofthe vapor mixture, and rate of introduction of the vapor mixture aremaintained at values such that at least a part of the reaction takesplace within the gaseous mixture at a position spaced from the innerwalls of the said tube thereby producing a suspension of oxidic reactionproduct particulate material, which deposits on the inner surface of thetube and in that said particulate material is consolidated to producesaid glassy deposit on the inner surface of the tube.
 2. The method ofclaim 1 wherein additional material is subsequently deposited on theouter surface of the tube.
 3. The method of claim 1 wherein the tube iscollapsed after the glassy deposit is produced and glass precursorreactant vapors are maintained in the tube during at least a portion ofthe tube collapse.
 4. The method of claim 2 or 3 wherein the said movinghot zone serves the dual functions of nucleation site for homogeneousreaction to produce particulate material and consolidation site forpreviously produced particulate material.
 5. The method of claim 3wherein the glass precursor reactants are flowed through the tube duringat least a portion of the collapse.
 6. The method of claim 5 wherein thetube collapse is initiated with reactants still flowing.
 7. The methodof claim 1 wherein there is interposed between the deposited materialwhich will ultimately form the optical fiber core and the tube, a layerof glass which acts as a barrier to impurities diffusing from the tubematerial to the deposited core material.
 8. The method of claim 7wherein the barrier layer acts as at least a portion of the opticalfiber cladding.